Comparison and Characterization of Colletotrichum acutatum the Main Causative Agent of Colletotrichum Leaf Disease (CLD) of Rubber (Hevea brasiliensis) with Newly Identified Causative Agent Colletotrichum siamense
Colletotrichum acutatum has been identified as the major pathogenic species of CLD in rubber. Recently, C. siamense was identified as another causative agent of this disease from Sri Lankan rubber plantations. The present study was focused to compare C. acutatum and C. siamense giving special reference to cultural and reproductive characteristics, fungicide sensitivity and pathogenicity. Comparison of new pathogens with already existing pathogens are required for understanding of new pathogen population. That knowledge will be important to improve management strategies, quarantine regulations and to protect crop avoiding a huge economical lose. C. siamense was isolated from symptomatic rubber leaves on to Potato Dextrose Agar (PDA) and C. acutatum was collected from department culture collection. Cultural characters were observed after 10 days of incubation. Reproductive characters were compared using slide culture technique. Conidia concentrations were detected using haemocytometer. Growth rates were measured along two perpendicular axis on PDA plates. Fungicide sensitivity was tested with 0.01-500 ppm carbendazim following poisoned food technique. Pathogenicity test was performed using 5×105 conidia/ml suspension on detached rubber leaves and Disease Index (DI) was calculated. Colonies were circular, smooth and cloudy with raised elevation, opaque nature and entire margin. Both upper and lower surfaces of C. siamense were light grey with orange conidiomata on the lower surface. C. acutatum produced white to light grey colonies which were initially salmon coloured in lower surface. Conidia in both isolates were aseptate one celled, hyaline, smooth-walled and granular with size of 14-16 μm×4-5 μm. C. siamense produced cylindrical, both ends rounded spores with 4×105 conidia/ml concentration while C. acutatum produced fusiform conidia with 2.08×107 conidia/ml upon 7 days of incubation. However, the growth of C. siamense in PDA was higher (1.60 cm/day) than the growth of C. acutatum (1.48 cm/day). Although C. siamense inhibited 100% of its growth even at 0.5 ppm of carbendazim, C. acutatum was extremely tolerant to carbendazim and couldn’t inhibit the growth even at 500 ppm. In the pathogenicity test, lesions were produced at all inoculation points but the size and type of lesions produced by both isolates were different. C. acutatum produced darker anthracnose-like watery lesions with 3.00 DI value and C. siamense produced slightly dark watery lesions with DI value of 2.00. According to the characters of both isolates, C. acutatum can be identified as a severe pathogen than C. siamense.